For what it's worth, I wasn't able to duplicate these ResourceWarnings (in Python 2.7.5 and 3.3.2 under Windows).

Taking a quick glance at the code, the only thing that jumps out at me is that formtools.wizard.storage.base.BaseStorage.get_step_files() opens temp files that get passed along (via current_step_files). I don't know enough about this code to say when or whether these file handles get closed, but I thought I'd mention it.

Basically, the explanation of this behavior is rather straightforward: at various steps, formtools is reopening files (in get_step_files) to provide them to the wizard form(s), a bit like a standard UploadHandler does it for data coming from a request to populate request.FILES (and then form files data).
If we look at the same process with standard forms, we can see that we don't explicitly close the files either (not an issue with InMemoryUploadedFile, not sure about TemporaryUploadedFile for bigger files). In standard form tests, it's easy to manually close files as we are opening them ourselves. It's different in formtools tests, as those files are reopened inside formtools code.

This leaves us with two paths:

We don't think that not explicitly closing files in this situation is a problem, and we should then simply silence the warnings (didn't try if it's possible or not) or live with them.

We do think that open files provided to forms with file fields should be explicitly closed at some point in the code (and not artifically in test code). But then the resolution is not in formtools, but in the form infrastructure itself.

Reopening, as the fix is causing failures. On Python 2 at least, __file__ can have the .pyc extension, so UPLOADED_FILE_NAME is not always equal to __file__. I guess the CI server is cleaning .pyc files before each run.